A Cool Solar Mystery

February
20, 2007: One pole of the sun is cooler than the
other. That's the surprising conclusion announced today by
scientists who have been analyzing data from the ESA-NASA
Ulysses spacecraft.

Ulysses
is the only ship in the NASA or European fleet capable of
flying over the sun's poles, a result of the spacecraft's
uniquely-tilted orbit. Its ability to study the sun's unexplored
polar regions is prized by solar physicists.

Ulysses'
first polar flybys in 1994 and 1995 revealed the asymmetry—"a
7 to 8 percent difference in temperature," says Ulysses
science team member George Gloeckler of the University of
Maryland. The measurement was both mysterious and a little
hard to believe. What would make the sun lopsided in this
way?

Right:
Ulysses over the sun's South Pole, an artist's concept. Credit:
ESA.

There's
still no definitive answer to that question, but now at least
researchers know the effect is real. Ulysses has returned
to the sun's South Pole in 2007 and "recent observations
show that the average temperature ... is virtually identical
to what we saw 12 years ago," says Gloeckler.

Taking
the sun's temperature is tricky business. The spacecraft can't
descend to the surface and insert a thermometer. Instead,
Ulysses samples the solar wind at a safe remove of 300 million
km. "We measure the abundance of two oxygen ions found
in the solar wind. The ratio O6+/O7+
tells us the temperature of the gas," explains Gloeckler.
He is the principal investigator of the instrument onboard
Ulysses that does this, the Solar Wind Ion Composition Spectrometer
or "SWICS."

According
to SWICS, the average temperature of the sun's polar wind is
about one million degrees C. But over one pole the wind is about
80,000 degrees cooler than over the other pole.

Researchers
believe the solar wind at Ulysses is telling them something
about polar conditions close to the surface of the sun. "The
solar wind comes from the poles," explains Arik Posner,
Ulysses Program Scientist at NASA headquarters. "The
sun's magnetic field opens up over the poles and allows some
of the sun's atmosphere to escape." These openings are
called "coronal holes," and the hot atmosphere that
rushes out is the solar wind.

Back
to the original question: What does the temperature difference
mean? "Perhaps the structure of the sun's atmosphere
over the two poles is different," he speculates.

We
have an analogy here on Earth. The stratosphere over the South
Pole is colder, on average, than the stratosphere over the
North Pole. The reason has to do with the uneven distribution
of land on Earth (most land is in the north) plus complex
atmospheric circulation patterns.

In
the case of the sun, the difference is not land but magnetism.
Apparently, something about the sun's north magnetic pole
keeps the solar atmosphere above it a trifle cooler. Proof:
The "cool spot" follows the north magnetic pole
when the sun's poles flip.

"The
sun's magnetic poles have reversed polarity since the 1994
flyby—an effect of the 11-year sunspot cycle," notes
Posner. Lo and behold, "the temperature asymmetry has
also reversed. So it appears to be a magnetic phenomenon."

When
Ulysses finishes its current South Pole flyby, it will proceed
to the other end of the sun for a North Pole flyby in early
2008. This will provide more clues to what's shaping up to
be a very cool solar mystery.

South
Pole Flyby (Science@NASA) -- the ESA-NASA Ulysses
spacecraft is flying over uncharted territory, the mysterious
South Pole of the sun.

Above:
Ulysses' third orbit around the sun carries it over
the South Pole in February 2007 and the North Pole in
early 2008.

Cold
Peril -- (Science@NASA) The NASA/ESA Ulysses spacecraft
is perilously cold as it begins a newly extended mission
to study the sun.

A
Star with two North Poles -- (Science@NASA) Sometimes
the Sun's magnetic field goes haywire. The Ulysses spacecraft
has discovered how the effects are felt throughout solar
system.

Solar
Flares on Steroids -- (Science@NASA) Solar flares
that scorch Earth's atmosphere are commonplace. But
scientists have discovered a few each year that are
not like the others: they come from stars thousands
of light years away. The Ulysses spacecraft is crucial
for pinpointing these "solar flares on steroids."